As technical development and demand for mobile devices has increased, the demand for secondary batteries as an energy source has increased dramatically. Among such secondary batteries, lithium secondary batteries, which exhibit high energy densities and low operating potentials, and have long cycle lives, and low self discharge rates, have been commercialized and are widely used.
Moreover, as interest in environmental issues has increased recently, there has been much research on electric vehicles (EV) and hybrid electric vehicles (HEV) and the like which are among the leading causes of air pollution. Although nickel-metal hydride (Ni-MH) secondary batteries are typically used as the power source for electric vehicles (EV) and hybrid electric vehicles (HEV) and the like, there has been active research into the use of lithium secondary batteries, which have high energy densities, high discharge voltages, and output stability, some of which have been commercialized.
Meanwhile, metal oxides such as LiCoO2, LiMnO2, LiMn2O4, or LiCrO2 are used as positive electrode active materials constituting positive electrodes in lithium secondary batteries, and materials comprising metal lithium, carbon based materials such as graphite or activated carbon, or silicon oxides (SiOx) and the like are used as negative electrode active materials constituting negative electrodes. Among the negative electrode active materials, although metal lithium was typically used early on, due to a phenomenon in which lithium atoms on the surface of the metal lithium grow and thereby damage a separator as a charge/discharge cycle proceeds, recently, carbon based materials are typically used.
Carbon based materials, in particular graphite, are reversibly lithiated/delithiated with a significant amount of lithium, up to one lithium per six carbon atoms, without degradation to the mechanical and electrical properties thereof.
The lithiation/delithiation reaction rate is closely dependent on the resistance related to material transport and charge transport processes. Lithium ions move to the surface of graphite through an electrolyte (solution resistance, RS), infiltrate a solid-electrolyte interface (SEI) layer (RSEI), and from the SEI, are inserted into peripheral locations of the graphite (charge transport resistance, RCT), and diffuse along spaces inside the graphite.
Since solid state diffusion of Li+ may be the rate determining step during high-speed charging and discharging, charge transport that results in LiC6 formation is limited. Concentration polarization and resistance polarization induce higher overpotential during high-current charging, and thus cell potential reaches the cut-off voltage before the graphite is completely lithiated. As graphite is lithiated with larger amounts of Li+, the electrochemical environment changes such that the insertion potential of Li+ decreases.
However, current lithium secondary battery charging and discharging methods simplify, rather than reflect, resistance differences dependent on the state of charge (SOC) of graphite. When, as described, charging and discharging is performed without reflecting the resistance differences dependent on the state of change of graphite, not only is the rate of charge/discharge not optimized, the durability of graphite based active materials is also adversely affected.
Thus, there is a need for developing a novel charging method capable of charging a lithium secondary battery at a current density corresponding to the resistance difference dependent on the state of charge of graphite.